@Pure Fun
If your main reason is to explore the northern Gulf of Siam in Dec. There is nothing wrong with it as Koh Chang can be reached from Bangkok in few hrs by bus/ferry.
However, if your trip is mainly island hopping and scuba diving then I would suggest you ought to look some where else in the Kingdom.
BTW, I am NOT associated with any travel agent/dive operator etc. We are in the same boat.
Avoid Phuket except the airport and the two bus terminals(1 and 2).
My suggestion:
Dec will be extremely busy in the Kingdom.
1. Early flight to Phuket, airport bus(B100.00) to T1, check if there is public transport to Khao Lak otherwise you need to pay B15.00 for a relative cheap transfer(Pink Truck) to T2. Koh Lak is a far more laid back destination than Phuket and some diving operators will arrange speed boat daily transfer to dive Similan.
en.wikivoyage.org
It was hit hard by the Tsunami in 2004.
2. Early flight to Krabi for Koh Lanta or Hat Yai for Koh Lipe. Transfer between the two islands is a bumpy 3hrs by speed boat. Diving out from Koh Lanta is quite expensive because of the travelling distance. Koh Lipe is tiny and for some reason attracts large number of backpacker. I have no problem with that as it is still my prefer way in travelling.
3. The daily dive boat out from Phuket in Dec should be extremely crowded.